The referenced article "Low-Frequency Generator with Low Non-linear or Harmonic Distortion", by Gerd Koetter, in the publication "FUNKSCHAU" 1978, issue No. 8, pages 109, 110, and particularly FIG. 5 of the article, discloses a tunable oscillator in which the frequency of oscillation is determined by a resistor. The particular oscillator there referred to comprises two operational amplifiers which are cascaded and connected in an oscillatory circuit. By use of a phase shift circuit in combination with the operational amplifiers, adjustable resistances can be used to vary the frequency of oscillation of the overall circuit over a wide range. By changing of the resistance of the respective resistor or resistors, the oscillator is tuned. Tuning ranges in the order of 2000:1 can be obtained, with extremely low distortion.
The adjustable resistors in the oscillator circuit usually use potentiometers or switchable resistors. Mechanical switches as well as potentiometers have disadvantages in that the contact resistance of the elements has to be considered and may vary from adjustment to adjustment; the contact resistance of the slider of a potentiometer must be included in the resistance which determines the frequency of oscillation. The contact resistance both of sliders as well as of switch contacts changes with age, use, corrosion of the contact surfaces, and the like. In order to obtain accurate reproducibility of resistances in a switchable resistance arrangement, it has been proposed to use contacts having noble metal coatings, for example gold contacts. Such types of contacts are expensive.